How to aggregate two lists if at least one element matches in MongoDB?

To aggregate two lists if at least one element matches in MongoDB, use the aggregation pipeline with $unwind, $group, and $addToSet operators to identify common elements between arrays and group documents accordingly.

Syntax

db.collection.aggregate([
    { $unwind: "$arrayField" },
    { $group: { _id: "$arrayField", documentIds: { $addToSet: "$_id" }, count: { $sum: 1 } } },
    { $match: { count: { $gt: 1 } } },
    // Additional pipeline stages to format results
]);

Sample Data

db.demo456.insertMany([
    { _id: 101, StudentName: ["Chris", "David"] },
    { _id: 102, StudentName: ["Mike", "Sam"] },
    { _id: 103, StudentName: ["John", "Jace"] },
    { _id: 104, StudentName: ["Robert", "John"] }
]);

{ "acknowledged" : true, "insertedIds" : { "0" : 101, "1" : 102, "2" : 103, "3" : 104 } }

Verify Sample Data

db.demo456.find();

{ "_id" : 101, "StudentName" : [ "Chris", "David" ] }
{ "_id" : 102, "StudentName" : [ "Mike", "Sam" ] }
{ "_id" : 103, "StudentName" : [ "John", "Jace" ] }
{ "_id" : 104, "StudentName" : [ "Robert", "John" ] }

Example: Find Documents with Matching Elements

db.demo456.aggregate([
    { $unwind: "$StudentName" },
    { $group: { _id: "$StudentName", combine: { $addToSet: "$_id" }, size: { $sum: 1 } } },
    { $match: { size: { $gt: 1 } } },
    { $project: { _id: 1, combine: 1, size: 1, combine1: "$combine" } },
    { $unwind: "$combine" },
    { $unwind: "$combine1" },
    { $group: { _id: "$combine", l: { $first: "$_id" }, size: { $sum: 1 }, set: { $addToSet: "$combine1" } } },
    { $sort: { size: 1 } },
    { $group: { _id: "$l", combineIds: { $last: "$set" }, size: { $sum: 1 } } },
    { $match: { size: { $gt: 1 } } }
]);

{ "_id" : "John", "combineIds" : [ 103, 104 ], "size" : 2 }

How It Works

• $unwind − Deconstructs the StudentName array into separate documents
• $group − Groups by student name and collects document IDs using $addToSet
• $match − Filters for names that appear in multiple documents (size > 1)
• The remaining stages reorganize the data to show which document IDs share common elements

Conclusion

Use MongoDB's aggregation pipeline with $unwind and $group operators to identify documents that share at least one common array element. The pipeline efficiently groups documents by matching array values and returns the aggregated results.